The present invention generally relates to a semiconductor memory device, and more particularly, to a nonvolatile transistor memory having a gate insulating assembly for trapping charges which includes two or more types of alternately laminated insulating thin films and a tunnel oxide film located on a semiconductor substrate.
Some conventional semiconductor readable and writable memory transistors, which store information by the accumulation of electric charges, have a MNOS (metal-film nitride-oxide film-semiconductor) construction. The minimum thickness of a nitride film of the MNOS device is 190 .ANG.. An oxide film is formed on the upper surface of the nitride film to achieve a relatively thin gate insulating film structure to cope with lower voltages and to obtain higher speed memory elements. However, the injection of holes from the gate electrode can lead to element deterioration due to the thin gate insulating film structure.
In one proposed memory device, a tunnel oxide film is provided on a silicone substrate for trapping an electric charge thereon, and a gate electrode is provided on the trap film. By application of a voltage upon the gate electrode, an electric charge is trapped on the trap film from the channel portion of the transistor device. After the gate voltage has been removed, the channel is retained in an on or off state according to an electric field resulting from the trapped electric charge.
In one type of device, a nitride film or mixed film is used as a trap film. In another type of device, an oxide film is provided on the trap film so that erasing and writing operations can be effected using a low voltage. The oxide film prevents the injection of holes from the electrode and the thrusting of electrons into the electrode. However, since a relatively high voltage is applied to the oxide film, defects occur as the writing and erasing voltage increases, and detrap of electrons and injection of holes are caused thus reducing the injection efficiency of electrons.